1. Field of the Invention
The present invention relates to an automatic speed control apparatus for enabling a vehicle such as an automobile driven by an internal combustion engine to run at a constant set speed. More particularly, the invention is concerned with an automatic driving apparatus of the type having a main intake passage leading to an engine, an auxiliary intake passage which directly connects a portion of the main intake passage upstream of a throttle value and a portion of the same downstream of the throttle valve, an auxiliary intake air control valve disposed in the auxiliary intake passage and adapted for controlling flow rate of air flowing through the auxiliary intake passage, and a fuel supply cut-off means which suspends the supply of fuel to the engine from a moment at which a condition is sufficed such that the engine speed is higher than a predetermined fuel supply cut-off engine speed while the throttle valve opening is minimum until a moment at which a predetermined fuel supply recovery condition is met, thereby regulating the running speed of the vehicle.
2. Related Art Statement
Hitherto, an automatic driving apparatus has been known which comprises a speed setting means for setting a desired vehicle running speed by way of a set switch, means for computing the offset of the actual vehicle speed from the set vehicle speed, and an electromagnetic throttle actuator to which electric power of a duty ratio is supplied and controlled in accordance with the computed speed offset so as to control the opening of a throttle valve and enable a constant running speed. Usually, the internal combustion engine combined with this type of speed control apparatus has a fuel injection controller which controls the rate of injection of fuel by way of fuel injection time TAU determined as TAU=K.Q/NE, where Q, NE and K represent, respectively, the intake air flow rate, engine speed and a constant. The fuel injection controller has two functions: namely, a fuel supply suspending function which suspends the fuel supply to the engine from a moment at which a condition is sufficed such that the engine speed is higher than a predetermined fuel supply cut-off engine speed while the throttle valve opening is minimum, and a fuel supply recovery function which recovers the fuel supply when a condition is met such that the engine speed has become lower than a predetermined fuel supply recovery speed, the engine speed has become lower than the fuel supply cut-off engine speed while the throttle valve opening is minimum, or the throttle valve opening is increased during the suspension of the fuel supply. The engine is also provided with an auxiliary intake passage which bypasses the throttle valve so as to directly connect portions of the main intake passage upstream and downstream of the throttle valve, and an idle speed control valve (ISC valve) provided in the auxiliary intake passage adapted to vary its degree of opening so as to control the flow rate of air in the auxiliary intake passage and thus maintain a predetermined engine speed during idling. The degree of opening of the ISC valve attained during idling is maintained even while the vehicle is in motion.
When a vehicle runs down a long descending slope, the vehicle having the vehicle speed control apparatus described above combined with an engine equipped with the above-described fuel injection controller and the auxiliary intake passage, the automatic speed control apparatus will operate to minimize the throttle opening, i.e., to fully close the throttle valve, because of the increasing the vehicle running speed due to inertia. If the engine speed in this state is higher than the aforementioned fuel supply cut-off engine speed, the fuel injection controller operates to stop the injection of fuel. As a result of the fuel supply cut-off, the engine output torque is drastically decreased from a point A to a point B on a solid-line curve as shown in FIG. 2, so that the vehicle is shocked undesirably. The vehicle speed is also decreased to a level below the speed set in the speed control apparatus as a result of suspension of the fuel supply. In this way, the speed control apparatus operates to control the throttle valve and recover the set vehicle speed. More specifically, the throttle valve is moved from the fully closed position to a greater degree of opening. This activates the fuel supply recovery function of the fuel injection controller so as to start the supply of the fuel. Consequently, output torque of the engine is cylically varied due to repetitive functioning of the fuel supply cut-off and fuel supply recovery functions. Such a cyclic variation of the engine output torque causes the driving system to vibrate, resulting in a phenomenon known as "surging" which imparts an unpleasant feel to the driver. In FIG. 3, curve (1) shows the change in speed of a vehicle as observed when the vehicle runs along descending slopes having gradients of -5% and -0.8% with the throttle valve almost fully closed. It will be understood that the vehicle speed changes in a vibratory manner due to repetitious stopping and starting of fuel injection.
In order to obviate this drawback, Japanese Patent Laid-Open Publication No. 135334 discloses a system in which, when the vehicle is running along a descending slope under the control of an automatic driving apparatus, the duty ratio of electric power supplied to the throttle valve actuator for actuating the throttle valve is controlled such that the periods of the fuel supply cut-off and fuel supply cycles are prolonged so as to reduce the frequency of these cycles, thereby eliminating any unpleasant shock which may otherwise be imparted to the driver.
This system effectively decreases the frequency of vibratory change in the engine output torque by virtue of the prolonged period of fuel supply cut-off and fuel supply cycles but is still unsatisfactory in that the shock itself cannot be completely eliminated because the cut-off of the fuel supply in effect causes a drastic change in the engine output.
Another problem is that the periods of the fuel supply cut-off cycle and fuel supply cycle, prolonged as a result of the duty-ratio control mentioned above, undesirably allow the vehicle to run at a speed which is offset to the higher side or lower side of the set vehicle speed. Consequently, the offset of the vehicle speed is increased to make the speed control unstable.
Such problems would be overcome if the fuel injection controller were so designed as to prohibit the cut-off of fuel supply whenever the vehicle is running on a descending slope while being controlled by the automatic driving apparatus. In such a case, however, the retarding effect known as "engine brake" becomes ineffective due to the fuel being supplied even when the throttle valve opening is minimized. As a result, the vehicle speed is increased as shown by curve (2) in FIG. 3. In addition, the supply of fuel under the condition where the engine torque is not demanded will cause "after burn" of the fuel which produces unfavorable effects such as over-heating of an exhaust gas cleaning catalyst and an increase in the rate of fuel consumption.
Through an intensive study on the problems described hereinbefore, the present inventors have found that the heavy shock produced when the fuel supply is cut-off and when the fuel supply is recovered is attributable to the presence of air flowing through an auxiliary intake passage, as will be understood from the following description.
The suspension of fuel supply essentially requires that the throttle valve be fully closed. Thus, the intake air flow rate is progressively decreased before the fuel is actually cut-off, and, when the cut-off of fuel supply is actually executed, the intake air flow rate has been reduced to a minimum level substantially the same as the flow rate q of air in the auxiliary intake passage. As explained before, the fuel injection controller is adapted to control the fuel injection rate in proportion to the intake air flow rate. This means that the fuel injection rate has been reduced to a level proportional to the intake air flow rate q at the moment immediately before the cut-off of fuel. The fuel injection rate is then drastically reduced to zero due to the fuel supply being actually cut-off. The fuel supply rate is thus changed non-linearly in an amount proportional to the flow rate q of the air flowing in the auxiliary intake passage. As a result, an abrupt change in the engine output torque causes a shock to a passenger in the vehicle.
When a condition for recovering the supply of fuel is fulfilled, the injection of fuel is again commenced and the rate of fuel supply is drastically increased from zero to a value proportional to the flow rate q of air flowing in the auxiliary intake passage. Consequently, the engine output torque is abruptly increased and a shock to the passengers results.